The present invention relates to an A/D conversion apparatus that can precisely convert an analog signal to a digital signal.
An A/D conversion apparatus converts an input analog signal to a digital signal as an output and uses a fast and precise A/D converter. The A/D conversion apparatus may use a transformer as a drive circuit to provide the analog signal to the A/D converter to make full use of the precise characteristics of the A/D converter. Relative to use of an amplifier, the use of the transformer as described above allows converting precisely an analog signal to a digital signal because it is not affected by non-linearity and/or circuit noise of the amplifier and then the analog signal is provided to the A/D converter without distortion. Besides, the use of the transformer allows providing the A/D converter with the analog signal as a differential signal, which reduces influence of higher harmonic wave, common mode noise, etc.
In the case of providing an analog signal to an A/D converter via a transformer, if the analog signal is getting closer to DC or a low frequency close to DC, it cannot provide the analog signal to the A/D converter via the transformer without distortion. Therefore, wideband analog to digital conversion requires a DC coupled drive circuit as well as the AC coupled drive circuit using the transformer.
FIG. 1 is a block diagram of a conventional A/D conversion apparatus that has AC and DC coupled drive circuits. An input analog signal is properly adjusted by an attenuator (not shown) and provided to a down-converter 51 of the A/D conversion apparatus 50 as an analog signal Sm. The analog signal Sm is also provided to a low bandpass filter 55 through a buffer amplifier 54.
The down-converter 51 multiplies the analog signal Sm and a local signal Sla to conduct frequency down conversion and provides an analog signal Sfd after the frequency conversion to one terminal of the primary winding of the transformer 53 via a gain block 52. The other terminal of the primary winding is grounded and the secondary winding is coupled to fixed terminals PTa1 and PTa2 of a signal selection switch 58.
The low bandpass filter 55 filters the analog signal Sm provided via the buffer amplifier 54 to remove an unnecessary high band component and provides the filtered analog signal Sfp to a differential amplifier 57 via a gain block 56.
The differential amplifier 57 amplifies the filtered analog signal Sfp and provides it to fixed terminals PTb1 and PTb2 of the signal selection switch 58.
The signal selection switch 58 has movable terminals PTs1 and PTs2 that are coupled to differential inputs of an A/D converter 60. A sampling clock generator 59 provides a sampling clock signal Ssp to the A/D converter 60.
The signal selection switch 58 selects one of the analog signals that are provided from the transformer 53 and the differential amplifier 57, and provides the selected analog signal to the A/D converter 60. Wherein if a semiconductor switch is used as the signal selection switch 58, there is a possibility that the analog signal will have distortion because the level of the provided analog signal may be too large as the analog signal provided to the signal selection switch 58 is amplified suitable for an input level of the A/D converter 60. Therefore mechanical relays that mechanically switch contacts are used as the signal selection switch 58 to avoid the distortion in the analog signal provided to the A/D converter 60.
In the A/D conversion apparatus 50 having configuration as described above, if the input analog signal is neither DC nor low frequency close to DC, the signal selection switch 58 switches the movable terminals PTs1 and PTs2 to the fixed terminals PTa1 and PTa2 respectively to provide the A/D converter 60 with the analog signal induced in the secondary winding of the transformer 53. This allows providing the A/D converter 60 with the analog signal without distortion so that the input analog signal is precisely converted to a digital signal. On the other hand, if the input analog signal is DC or low frequencies close to DC, the signal selection switch 58 switches the movable terminals PTs1 and PTs2 to the fixed terminals PTb1 and PTb2 respectively to provide the A/D converter 60 with the analog signal provided from the differential amplifier 57. Therefore, even the input analog signal is DC or the low frequencies close to DC it provides the digital signal according to the input analog signal.
But there is a possibility that the A/D conversion apparatus 50 shown in FIG. 1 may not provide the analog signal to the A/D converter 60 because repetitive signal switching leads to degradation of the contacts since it uses the mechanical relays as the signal selection switch not to distort the analog signal. Besides, it can not realize fast signal switching.
Then, not to use the mechanical relays, an A/D conversion apparatus 70 shown in FIG. 2 has AC-coupled and DC-coupled drive circuits that have the respective A/D converters 71 and 72. Note that blocks in FIG. 2 corresponding to those in FIG. 1 are labeled the same.
If the AC-coupled and DC-coupled drive circuits have the respective A/D converters as describe, it solves problems on the contact degradation and the signal switching time, etc. But use of a plurality of A/D converters leads to high cost of the A/D conversion apparatus 70.
Besides, in case of using the differential amplifier 57 as shown in FIGS. 1 and 2, the distortion characteristics of the differential amplifier 57 is worse than those of a transformer. Therefore, even if a high-precision A/D converter is used, accuracy of the analog to digital conversion is determined by characteristics of the differential amplifier 57 and then it cannot convert the analog signal to digital signal with high precision.